1. Field of the Invention
This invention relates generally to electrochemical cells and methods of generating electricity using such cells and, more particularly, this invention relates to an electrochemical cell utilizing an alkaline metal or other reactive metal anode and an aqueous electrolyte.
2. Description of the Prior Art
Electrochemical cells utilizing a reactive metal anode, a nonreactive, electrically conductive cathode and an aqueous electrolyte are well known. Such cells are described in, for example, Rowley U.S. Pat. No. 3,791,871 (Feb. 12, 1974), Tsai et al U.S. Pat. No. 3,976,509 (Aug. 24, 1976) and Littauer et al U.S. Pat. No. 4,007,057 (Feb. 8, 1977), all of which are assigned to Lockheed Missiles & Space Company, Inc., Sunnyvale, Calif. or Lockheed Aircraft Corporation, Burbank, Calif. The respective disclosures of the above-identified patents are hereby incorporated herein by reference.
The cell disclosed in Rowley U.S. Pat. No. 3,791,871 typifies prior electrochemical cells utilizing a reactive metal anode and an aqueous electrolyte. In the cell of the Rowley patent, the anode comprises an elemental alkali metal such as sodium or lithium, and the electrolyte comprises an aqueous solution of sodium hydroxide or lithium hydroxide, respectively, produced by the electrochemical reaction of the anodic metal and water.
The anode of the Rowley patent is coated with a thin film of nonreactive, water soluble material which is not electrically conductive. Preferably, the film is the natural hydrated oxide which forms on the metal surface as it is exposed to humid air. However, other suitable water soluble insulators may be used for the film. The film is porous and allows transport of aqueous electrolyte to the anode and transport of reaction products away from the anode.
A cathode comprising a nonreactive, electrically conductive metal is spaced from the anode.
The electrolyte of the cell disclosed in the Rowley patent is formed by the electrochemical reaction of water and the anodic metal. Thus, in the Rowley cell, water is introduced to the cell at a restricted rate and brought into direct contact with both the cathode and the anode. The water dissolves at least a portion of the soluble film on the anode, resulting in the production of a hydrated hydroxide of the anode metal, plus heat. As the reaction proceeds, useful electrical power is produced.
The anode and the cathode are not in direct metallic contact, but circuit connections are made at each of the cathode and anode for drawing electrical power from the cell.
The alkali metal of the anode is highly reactive with water. This reactivity decreases as the concentration of metal hydroxide in the electrolyte increases. Thus, prior electrochemical cells of the Rowley type operate at electrolyte concentrations of the anode reaction product (e.g. NaOH or LiOH) well below saturation. For example, such cells usually operated at a maximum of about 60-80% of saturation, due to the tendency of alkali metals to passivate at higher electrolyte anodic metal hydroxide concentrations.
Additionally, prior alkali metal cells of the Rowley type (i.e. with unsaturated aqueous electrolytes) typically operate at relatively low temperatures of about 25-35.degree. C. (with an absolute maximum of about 50.degree. C. for elemental lithim anodes) due to the characteristic tendency of such cells to exhibit thermal runaway and, ultimately, to explode when operated at higher temperatures.
Thus, one major disadvantage of prior alkali metal/aqueous cells is the inability to operate at elevated temperatures due to the metastable control characteristics of such cells. In many environments, such as in aircraft, submarine or torpedo applications, where the ambient temperature is only slightly lower than the cell operation temperature, large, heavy heat exchange equipment is required in order to maintain the temperature at a constant level.
Additionally, such inherently unstable cells create complicated control problems and are inherently unsafe.
Since relatively high sodium hydroxide or lithium hydroxide concentrations must be avoided in prior cells to maintain a desired level of power output at operation temperatures, any excess aqueous lithium or sodium hydroxide must be removed, requiring inconvenient precipitants or ion exchange equipment. Also, since electrolyte concentration gradients are inherently present in such cells, the anodes tend to wear unevenly, resulting in inconvenient evolution of hydrogen gas and non-uniform voltages.